Anthony Sampson: These were not romantic buccaneers, but the lawless heirs of the apartheid years

It remains a thriller, but it is not a corny epic about intrepid white daredevils confronting black dictators

The story of Mark Thatcher's arrest in Cape Town, and the trial of suspected mercenaries in Zimbabwe and Equatorial Guinea, is so like a far-out thriller about the Dogs of War that its hard to realise its full diplomatic significance.

The story of Mark Thatcher's arrest in Cape Town, and the trial of suspected mercenaries in Zimbabwe and Equatorial Guinea, is so like a far-out thriller about the Dogs of War that its hard to realise its full diplomatic significance.

The characters and situations seem fictional stereotypes: an ex-prime minister's son with a dubious history arrested in Cape Town; his friend the Old Etonian mercenary Simon Mann, imprisoned by the black villain Mugabe and convicted yesterday by a court in Zimbabwe; the Afrikaner accomplice Nick du Toit facing the death sentence in a brutal dictatorship Equatorial Guinea; an international network, including the crudely coded "Scratcher" and "Smelly", and allegedly the ex-jailbird Lord Archer.

They all appear to belong to an old-fashioned adventure story about white adventurers intervening in the Dark Continent to safeguard the interests of Western businessmen.

But the truth is more interesting, and much less racial: the real reason for Pretoria's intervention was finally to move against the networks of mercenaries which are the most dangerous legacy of the previous apartheid governments, and have been the most difficult to overcome. And Thatcher's arrest comes at a time when the South African and British governments - including the intelligence services - are looking for closer friendship, symbolised by the arrival of the Foreign Secretary Jack Straw in Cape Town just afterwards, with a team of senior ministers, to try to improve the poor relations between Tony Blair and President Mbeki since the last Commonwealth meeting.

To understand the South Africans' problem, we have to look back to the last years of the apartheid governments in the 1980s when they unleashed secret troops across the continent to enforce their foreign policy. They provided the training ground of many of the freelance mercenaries today.

The South African intelligence services under President Botha, often encouraged by the CIA during the Cold War, financed and armed groups of ruthless hit squads: not only to capture and assassinate the ANC within South Africa, but also to undermine the official governments in neighbouring countries, including Angola and Mozambique, by supporting the rebel armies of Unita and Renamo.

Inside South Africa, the apartheid government soon lost control of many of its own troops, which indulged in growing atrocities - as later documented by the Truth and Reconciliation Commission. And reckless and murderous bands, including some psychopaths, provided the breeding ground for mercenary armies which could soon make more money by working for commercial companies defending minerals and oil across the continent.

South Africa became the world's chief base for mercenary activity; while weak and corrupt governments across the continent were easy targets for rebel leaders, backed by big-business interests, which could deploy mercenaries against the ill-trained local armies.

It was this dangerous legacy which the ANC government inherited 10 years ago. The continued presence of mercenaries threatened the effectiveness of its own army and intelligence services, while other African states constantly feared intervention from free-booting groups flying up from the south. President Mbeki, after he took office five years ago, wanted to establish his country's leadership and reputation in the rest of Africa, and he was determined that South Africa must no longer be seen as the mercenary capital of the world.

His task was harder because many of the mercenary groups had British connections and an outward respectability, while British governments sometimes had an ambivalent attitude towards mercenaries, who could go to places where its own soldiers could not. The ambivalence was shown in Sierra Leone, where the British High Commissioner authorised the use of the company Sandline to protect diamond interests - a company founded by two well-connected Englishmen, Simon Mann and Tim Spicer.

Mbeki did not want to jeopardise South Africa's good name as a safe place for British investors and expatriates, and when Mark Thatcher came to live in Cape Town, he was welcomed. When Lady Thatcher visited South Africa in January 1990, she called on Mbeki, bringing Mark with her, and the meeting was very friendly (Mbeki assured me) despite Lady Thatcher's previous hostility to the ANC.

At the same time, Mbeki's government had very good relations with British intelligence which had been forged before 1990 when MI6 was helpful to the ANC in exile in Lusaka - including the new minister for intelligence, Ronnie Kasrils. And Kasrils, as a former buccaneering freedom fighter who had come up against the full ruthlessness of the apartheid police, had a special motivation for confronting the mercenary menace.

So in March this year, it made good sense for the South African government to tip off Mugabe's police that mercenaries led by Simon Mann were arriving in Zimbabwe to buy arms for a coup in Equatorial Guinea, and to enable them to be arrested. It is still unclear whether the tip-off originally came from American or British intelligence; or who was actually backing the coup - though some fingers are pointing towards French oil companies which would have a natural interest in concessions in the oil-rich state.

For Mbeki, it was important to show that his government had finally made a clean break with the apartheid habits of intervening recklessly in other African states, while taking the lead in African solidarity. However nasty the regime in Equatorial Guinea, he needed to be friendly with a country with so much oil, for oil has become an important currency in Africa.

At the same time, Mbeki's government is very unlikely to allow the extradition of Mark Thatcher to Equatorial Guinea, which could be a fate worse than death, while even including death. Mbeki wants to co-operate with African states, but also to show that the South African legal system (which forbids the death penalty) is far superior to others.

So far, the arrest of Mark Thatcher has appeared to strengthen Mbeki's position: the image of the ex-prime minister's son being taken off by a black policeman has not evoked the racial overtones which would have been immediately aroused if it had happened in Zimbabwe; and the British Government has shown no desire to support Thatcher's friend Simon Mann in Zimbabwe, despite his appeals. But it remains to be seen how far the British and the American governments are prepared to grapple with the wider problems of confronting mercenaries - at a time when they have become so powerful in Iraq.

The extraordinary story remains a thriller, but it is not a corny epic about intrepid white daredevils confronting black dictators: it is a more serious story about the need to move against lawless mercenaries operating beyond the control of governments, who need to be checked before they wreck the prospects of peace in Africa and elsewhere.